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If your drain plug has become cross-threaded causing it to leak, there really is no fast fix to recover the threads to the original size and consider the fix permanent. First determine if the damage is to the plug, the casing or both. On the transmission, the plug is steel and the casing aluminum, so I would expect the damage to be to the casing. It’s still a good idea to replace the plug, but determine your repair method before you buy anything. There are many methods to repair damaged threads, but only a couple of ways to really fix this that it remains fully serviceable.

Trying to retap the hole straight to original size and use liquid sealant, thread-building epoxies and the like are just temporary fixes. They would work as well as wrapping the plug with plumbers’ teflon pipe tape. Like tape, they would have to be repeated with every oil change. If it is just a minor leak, you could just tape the plug or use a sealant, though, unless very clean of oil, the teflon pipe tape would work better.

My suggestion as the best method of permanent repair would be to use an oversized drain plug. You would need to drill-out the original threads, bringing the hole to a larger size and re-tap it for a slightly larger plug. Square- or socket-head plugs of all sizes are available and the difference in size will not be noticeable. It would also fit as the original, with only one set of threads to be concerned about leaking (see comments on the other repairs to follow).

If you are determined to retain all original parts on the car, then using a Helicoil or shell insert are the easiest and least expensive ways to use the original size plug. Helicoils come as a kit for the specific size bolt and tap you want to work with. It includes an oversized drill, corresponding tap and spring-like coil that rebuilds the threads to your original size.

A shell kit works similarly, but is heavier. It looks like a pipe or sleeve threaded on both the inside and outside. It requires a somewhat larger hole drilled into the casing than the Helicoil, which is tapped to match the outer thread size of the sleeve and it is the inserted and set with lock-tite or the like. The inside is already tapped to match your plug. Sleeves tend to be tighter than coils as they can be tapped to a tighter (or "interference") fit that you cannot do with the coils. They are less likely to loosen or leak with use if done right.

I have used sleeves and helicoils successfully on various applications both on the cars and in industrial factories. I had Helicoils in place to repair bugged spark plug threads in the past (which is what I think were developed for back in the 50’s), so sealing is not so much the problem, and have them on my MG in the bolts for the transmission motor mounts. The steel bolts on soft aluminum casings had worked themselves loose and damaged the threads over the years. The advantage to both these methods is that you can use the original replacement plug. Both, however, add a second set of threads which are another area for leakage as the fix gets older. Where sealing is a concern, using lock-tite or other thread locking adhesive to set the coil would definitely help, but keeping the tap tight and getting the threads clean of oil and dirt is the real trick to making it work.

For all 3 of these methods, I would invest in a collar for the drill bit to prevent it from getting too deep into the gearbox and chance damaging the internals as well as flushing the gearbox with light oil to ensure all chips and dirt are removed before refilling. That the plug is in the bottom, though you have to work lying your back on the floor or with the car on a lift, it is less likely any chips will get too far into the gearbox.

I am traveling out of the country, so cannot check the spare trans in my garage or have a chart with me to identify the plug bolt size, so cannot advise the oversized bolt or helicoil kit you might need. Bringing the old one to a NAPA or other automotive, truck or tractor machine tool and parts shop (or even some of the better hardware stores), they should be able to help you with what you need.